Vaccinia virus is used to vaccinate persons to prevent disease with smallpox. Compared with other live virus vaccines, the smallpox vaccine is associated with frequent side effects including fever, sore or swollen arm, headache, and fatigue. In a recent trial of persons who received the smallpox vaccine, about one-third of persons reported taking time off from work or school because of these side effects. Other, more serious, side effects have also been reported including development of multiple lesions outside the vaccination site (generalized vaccinia), inflammation of the brain (postvaccinia encephalitis), severe infection of the skin in persons with a history of eczema (eczema vaccinatum), and disseminated vaccinia virus infection in immunocompromised persons (progressive vaccinia). Relatively few studies of vaccine-induced immune responses have been done in the modern era using contemporary techniques. While it is known that vaccination induces production of antibodies to the smallpox vaccine virus, and induces cytotoxic T cells that can kill smallpox vaccine virus-infected cells, production of circulating cytokines in persons receiving the smallpox vaccine virus has not been studied. Cytokines are proteins that are produced by white blood cells and secreted into the bloodstream that are important for trafficking of white blood cells in response to inflammation. We hypothesize that induction of cytokines is associated with some of the side effects of smallpox vaccination. It is also uncertain just how frequently the smallpox vaccine virus spreads from the site of inoculation. Few studies have been done with molecular techniques to look for smallpox vaccine virus in the bloodstream or at other sites in the body using sensitive techniques such as polymerase chain reaction (PCR). We are collecting serial blood samples and throat swabs from healthy persons who are receiving the smallpox vaccine. We are measuring serum cytokines and looking for the smallpox vaccine virus (and virus DNA) in the bloodstream and throat of these persons. Identification of specific cytokines induced after vaccination may help to explain the pathogenesis of certain side effects associated with smallpox vaccination and might suggest new ways to modify some of these side effects. We are evaluating different methods of detecting the smallpox vaccine virus in clinical specimens, including sensitive cell culture methods and PCR. Evaluation of different diagnostic tests for smallpox vaccine virus may help to aid early identification of the virus in clinical specimens.